A “colorant” means in this document an independent variable with which an output device can be addressed. A “colorant value”, denoted as c, is an independent value that can be used to control a colorant of the output device. The colorants of an offset printing press, for example, are the offset printing inks. It is customary to express the range of physically achievable values for the colorants of a device in %, which means that usually the colorant values range from c=0% to c=100%. In graphic arts, colorant values are often called dot percentages. An output device or printing device with n colorants, wherein n≧1, will also be called below a “printer” or an “n-ink process”. The output device may be a proofing device with a cyan (C), a magenta (M), a yellow (Y) and a black (K) colorant.
To put a color output device in a standard state, a calibration procedure is applied. In fact, an output device can drift away from its standard state; e.g. changes in room humidity or use of a fresh supply of ink may cause a printer to produce different color. The objective of device calibration, therefore, is to bring a device back to a known, standard state, so that it produces predictable color every time it receives the same input colorant values. To calibrate a printer, typically a calibration target is printed by the printer and measured. If the measurements indicate that the printer has drifted away from its standard state, calibration curves are calculated from the measurement results to correct for this drift. A calibration curve transforms a colorant value to another colorant value. We refer to patent application EP 1 083 739 for more information on calibration, color gamut and other relevant terms.
For some color output devices it is advantageous to apply ink limitation in the calibration step. In this way it is possible to reduce the influence of visually disturbing artifacts, that may occur if too much ink is laid down on the receiving substrate. For ink-jet printers for example, the ink can bleed significantly, especially if the receiving substrate upon which the ink is deposited is some low quality paper such as newsprint, i.e. paper used for newspapers. Bleeding normally gets more important with increasing colorant values, i.e. with increasing ink amounts. Other special effects such as coalescence may also be reduced or avoided by limiting the ink amount. All these visually disturbing effects are referred to in this document as “artifacts”. On the other hand, for some printers there is no or no significant gamut increase beyond specific colorant values (the gamut is the delimited region in color space of the colors that are physically realizable by a given printer). Further, reducing the amount of ink decreases the drying time of the receiving substrate. Because of all these factors, applying ink limitation is important.
U.S. Pat. No. 6,371,609 to Oikawa teaches an inkjet recording apparatus that deals with the problem of ink spread. A solution that is disclosed in this patent is to eject a liquid in a separate row of nozzles that reacts with ink that has been ejected by another row of nozzles in the same print head.
U.S. Pat. No. 7,050,196 to Piatt et al. specifically deals with limiting the maximum amount of a colorant. According to this patent a maximum amount of colorant is determined by means of a test pattern that includes specific patterns for measuring bleed between colors, reverse text fill, paper cockle and ink showing through the back side of a paper.
A problem is that, up to now, artifacts are evaluated manually on printouts. This can be very time-consuming if the evaluation has to be repeated for many different ink limitation settings. Therefore, an improved method is needed.